Immunorective corticotropin-releasing factor (CRF) was identified and measured by radioimmunossay in rat thymic extract. Serial dilutions of thymic extract paralleled hypothalamic extract and synthetic CRF standard curves. CRF extracted from rat thymus eluted in the position of synthetic rat CRF on Sephadex G-25 column. Thymus-derived CRF was biologically active as demonstrated by stimulating adrenocorticotropic hormone secretion in dispersed rat anterior pituitary cells. These findings indicate the presence of authentic CRF in rat thymus, further supporting the concept of an integrated regulation of the neuroendocrine-immune responses to environmental stimuli.